1. Field of the Invention
The invention relates to ignition control in an internal combustion engine.
2. Description of the Related Art
A three way catalyst is widely used to remove harmful substances contained in exhaust gas from an internal combustion engine (i.e., exhaust emissions), such as carbon monoxide (CO), hydro carbon (HC), nitrogen oxide (NOX). A catalyst of this type however needs to be heated up to a specific activation temperature to utilize its purification capacity sufficiently. In other words, when its temperature is below that activation temperature, some harmful substances, particularly CO, can not be removed sufficiently. To counter this, therefore, various methods have been proposed and employed in which the timing of igniting an air-fuel mixture within each combustion chamber (will be referred to as “ignition timing”) is delayed so as to increase the temperature of exhaust gas (will be referred to as “exhaust temperature”) and thereby accelerate the activation of the catalyst.
Meanwhile, there is fuel for internal combustion engines which is less volatile than ordinary fuel. Hereinafter, such fuel will be called “heavy fuel” and ordinary fuel will be called “light fuel.” When heavy fuel is used, therefore, if the ignition timing is delayed in the same manner as when light fuel is used, it would result in reduced fuel economy, misfires, generation of insufficient torque, and so on. To avoid such drawbacks, a known method has been used to set the ignition timing in accordance with the engine speed (see Japanese laid-opened patent application No. 10-47039).
According to this method, more specifically, if the engine speed is above a prescribed value, it is determined that light fuel is being used and the ignition timing is therefore delayed. If the engine speed is below the same prescribed value, conversely, the ignition timing is advanced. As well as this example, there is another known method of identifying such a fuel type. According to this method, whether light or heavy fuel is used is determined based on the maximum value of the engine speed or its approximate value detected when the coolant temperature is within a specific range and the engine is idling after engine start (see Japanese laid-opened patent application No. 2000-205029).
In either method, however, it takes some time before the above-described delay of the ignition timing is actually carried out. Therefore, the catalyst can not be warmed quickly, and more importantly, it is difficult to reduce exhaust emissions, especially HC, during an engine start operation to the extent necessary to comply with vehicle emission regulation requirements that have recently become stricter.